The present invention relates to a process for producing 2,6-naphthalenedicarboxylic acid together with trimellitic acid by oxidation of 2,6-diisopropylnaphthalene (hereinafter referred to as "2,6-DIPN").
2,6-naphthalenedicarboxylic acid is useful as a starting material for polyethylene naphthalate, polyester, polyamide, etc. which are useful as a material for films and synthetic fibers having excellent heat resistance and mechanical properties, while anhydrous trimellitic acid is useful as a starting material for heat-resistant plasticizing ester, polyimide resins, curing agents for epoxy resin, coating materials and stabilizers.
Conventionally, trimellitic acid was synthesized by oxidation of pseudocumene with chromic acid or oxidation of rosin with nitric acid, but at present it is more industrially produced by air-oxidation of pseudocumene.
As a process for producing 2,6-naphthalenedicarboxylic acid (hereinunder referred to as "2,6-NDCA"), a method of oxidizing 2,6-dialkylnaphthalene such as 2,6-dimethylnaphthalene and 2,6-diisopropylnaphthalene in a solvent of acetic acid with molecular oxygen in the presence of a catalyst comprising cobalt and/or manganese, and bromine has been proposed (e.g., Japanese Patent Publication No. 48-27318 (1973), Japanese Patent Application Laying-Open (KOKAI) No. 60-89445 (1985) and Japanese Patent Application Laying-Open (KOKAI) No. 60-89446 (1985)).
According to such a method, since a large amount of bromine is used as compared with that of a heavy metal(s) in a mixed catalyst in order to obtain 2,6-NDCA with high yield, not only the alkyl group of 2,6-dialkylnaphthalene is oxidized but also the naphthalene ring is brominated, thereby producing various kinds of bromonaphthalenedicarboxylic acids as by-products as well as 2,6-NDCA. Since the physical and chemical nature of these bromonaphthalenedicarboxylic acids resemble those of 2,6-NDCA, it is very difficult to separate those bromonaphthalenedicarboxylic acids from 2,6-NDCA for purification. Therefore, removal of by-product bromides from crude 2,6-NDCA is the most important in producing 2,6-NDCA, as described in detail, for example, in Japanese Patent Publication No. 56-3858 (1981). Resins such as polyethylene naphthalate obtained from 2,6-NDCA contaminated by remaining bromides have a low softening point, and it is difficult to produce a film, fiber, etc. having high heat resistance and good mechanical properties out of such a contaminated 2,6-NDCA. In the above-described methods for producing 2,6-NDCA, aldehydes and colored substances having unknown structures are undesirably produced as by-products as well as bromonaphthalenedicarboxylic acids. These by-products are also difficult to separate and remove from 2,6-NDCA. Therefore, a conventional process for producing 2,6-NDCA requires many steps for separation and purification of 2,6-NDCA.
For example, in the method for producing 2,6-NDCA disclosed in Japanese Patent Publication No. 56-3858 (1981), 2,6-NDCA is obtained by oxidizing 2,6-dimethylnaphthalene with molecular oxygen in the presence of a catalyst comprising bromine, cobalt and manganese. Since a large amount of bromine is used as compared with that of cobalt and manganese (the atomic ratio of bromine to cobalt and manganese is 1.7), 1,000 to 2,000 ppm of bromine is contained in the crude 2,6-NDCA which is separated from the reaction mixture and 10 to 40 ppm bromine is still contained even after purification.
As described above, in the conventional processes for producing 2,6-NDCA using a large amount of bromine, a large amount of bromonaphthalenedicarboxylic acids, aldehydes, colored substances having unknown structure are produced as by-products. Therefore, many complicated steps of purification are required for producing high-purity 2,6-NDCA to remove these by-products from the crude 2,6-NDCA, so that the conventional processes are unsatisfactory. Particularly, removal of bromides which have a fatal influence on resin products obtained from 2,6-NDCA is difficult. Thus, it is extremely difficult to produce 2,6-NDCA of a satisfactorily high purity by a conventional process.
As a result of researches by the present inventors on a process for producing high-purity 2,6-NDCA, in particular, 2,6-NDCA substantially free from bromine, it has been found that if the atomic ratio of bromine to cobalt and/or manganese in the catalyst is reduced much more than in a conventional process, in a process for producing 2,6-NDCA by oxidizing 2,6-DIPN with molecular oxygen in the presence of a catalyst comprising cobalt, manganese or a mixture thereof and bromine, production of by-products, in particular, production of bromides such as a bromonaphthalenedicarboxylic acid is suppressed, thereby producing 2,6-NDCA with high purity together with high-purity trimellitic acid and, in addition, the recovery and regeneration of a catalyst of a heavy metal(s) is facilitated. The present invention has been achieved on the basis of this finding.